Story highlights

The march has been peaceful so far, with thousands of police deployed along the route

The political instability comes as Pakistan fights a bloody insurgency

Thousands of anti-government supporters led by two of Pakistan's most influential figures have joined a march to Islamabad to demand the resignation of the country's prime minister, prompting a major security operation.

By early Friday, an estimated 15,000-20,000 people had joined cricketer-turned-politician Imran Khan on the outskirts of Lahore in Pakistan's northwestern Punjab region. The procession took to the roads in cars, trucks and buses to make the 230-mile (370-kilometer) journey to the capital. The march started on Thursday as Pakistan celebrated Independence Day.

Khan, the former Pakistan cricket captain and leader of the Tehreek-e-insaaf (PTI) political party, has called on Prime Minister Nawaz Sharif to stand down amid claims of vote rigging during last year's election, a claim Sharif has rejected.

'Huge security operation'

The huge convoy was joined by followers of outspoken cleric Tahir ul Qadri, who called on his supporters to join what he described as a "revolution march." Qadri, who led protests against Pakistan's government last year that brought the capital to a standstill, has accused the Sharif government of corruption and campaigned for more to be done for the country's poor.

The march, which has been peaceful so far, prompted a huge security operation across the country, with tens of thousands of soldiers and police deployed along the route to Islamabad, while massive shipping containers and reels of barbed wire have been used to seal off many roads and highways.

Next stop Islamabad – Followers of Tahir ul Qadri escort a vehicle carrying the cleric through Pakistan's eastern city of Lahore on August 14.

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No entry – Main roads into the Islamabad, Pakistan are blockaded with shipping containers to prevent marchers from entering some areas of the capital.

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Calm before the storm? – Pakistani policemen stand alongside a roadblock of shipping containers near the Lahore home of cleric Tahir ul Qadri on August 13 as the country prepares for a massive anti-government march.

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Road-blocked – Pakistani policemen block the motorway which links Lahore and Peshawar to the capital Islamabad on August 13.

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Barbed response – Soldiers roll out barbed wire as they patrol the "red zone" area of Islamabad on August 13.

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No-go area – An excavator shifts sand into shipping containers placed by security forces as roadblocks on a street in Islamabad on August 13.

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V for victory – Supporters of cleric Tahir ul Qadri flash the victory sign outside his home in Lahore on August 13.

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Khan appeal – Cricketer-turned politician Imran Khan (C) appeals to supporters as he speaks with the media in Lahore on August 12.

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However, many Qadri supporters joined the march armed with sticks and improvised shields. Last week, at least five of his supporters were killed during clashes with security forces in the country's northwestern Punjab province.

Capital gathering

Organizers from both camps expect thousands more to join the procession as they get closer to the capital. A Khan representative told CNN Thursday that they anticipated around 100,000 people would be taking part. They are expected to assemble in central Islamabad, where security has been especially tight, with routes to many government buildings and embassies blocked by containers and guarded by army personnel, who assumed responsibility for the city's security.

Late Thursday, the country's interior minister Chaudhry Nisar Ali Khan told a press conference that the government had officially permitted the march to go ahead on the condition that protesters don't cross into the capital's "red zone" -- a tightly controlled area containing many government buildings.

So far, crowds in the capital have been small, but they're expected to increase significantly as the march draws nearer later Friday.

On Wednesday, in a rare address to the country, Prime Minister Sharif refuted accusations of electoral rigging and stressed that Pakistan has moved forward over the past 14 months in terms of "currency value, foreign investment and economic growth." In an apparent attempt to appease Khan's demands for an investigation into electoral fraud last year, Sharif offered to form a commission consisting of three Supreme Court judges to investigate the claims.

But at a press conference shortly after the prime minister's address, Khan said the government's response had arrived too late in the day and that he was now officially calling for the resignation of Sharif.

The political instability comes at a time when Pakistan is waging a bloody war against militants -- particularly in the restive tribal regions along its border in Afghanistan.

On Thursday two airbases in the southwestern city of Quetta in Balochistan Province were attacked by insurgent gunmen wearing suicide vests. But by early Friday, security forces repelled the attack, killing 10 terrorists and capturing another five, according to Imran Qureshi, the Superintendent of police in the city.

There was no immediate claim of responsibility for the attack. But, senior police officers suspect Islamist militants could be behind the two coordinated attacks. Sarfaraz Bugti, the Home Minister Balochistan, told CNN the dead gunmen were Uzbeks.